As one of the most important risk groups for HIV infection 82 intravenous drug addicts were tested in December 85/January 86 and 159 in December 86/January 87 for HIV antibody when entering a detoxification programme of our drug addition outpatient clinic. All these drug addicts had used intravenous drugs, mostly opiates. Serum antibodies to HIV were found in 7 (8.5%) of the first sample and in 23 (14.4%) of the second sample. Among seronegative drug addicts a significantly higher use of sterile needles was found than among HIV seropositive drug addicts. Among the seropositive drug abusers a history of intravenous drug abuse outside Austria was found more often than in seronegative drug addicts. The incidence of HIV antibodies in i.v. drug addicts in Vienna appears low in comparison with figures in the Austrian Tyrol (44%), Scotland (33%), Italy (53%) and Switzerland (53%), but similar to England (10%). The low frequency in Vienna could be explained by a significantly higher use of sterile equipment. Furthermore, at the first test period a change in abuse behaviour was found; an increasing number of patients was taking oral opiates exclusively, or reduced intravenous drug intake.